The present invention generally relates to fiber preforming methods and tooling used to form fiber preforms. More particularly, this invention relates to screen tooling for a fiber preforming process and a coating system therefor, in which the coating system inhibits sticking of fiber preforms to the tooling and promotes the tooling life of the screen tooling.
Various methods are known for forming fiber preforms that are suitable for producing fiber-reinforced articles, such as glass-reinforced polymer composite beds for pickup trucks. One such method is referred to as directed fiber preforming, which makes use of a perforated screen tooling having a surface whose shape corresponds to that of the desired fiber-reinforced article. In this process, reinforcement fibers (typically glass fibers) are sprayed onto the surface of the screen tooling and held on the tooling surface by a vacuum drawn through the tooling. The fibers are then bonded together with a binder to yield a porous fiber preform having a fixed shape corresponding to that of the tooling surface. The binder may be precoated on the fibers, simultaneously sprayed with the fibers onto the tooling, or sprayed on the fibers while the fibers are held on the tooling under vacuum. After curing, the fiber preform is removed from the screen tooling and placed in a suitable mold into which a resin is injected to infiltrate the preform. The resin is then cured to yield the fiber-reinforced article.
The release of the preform from the screen tooling is a processing issue for directed fiber preforming processes. For the preform to maintain its integrity, it must release cleanly from the tooling after the binder has been cured. However, in the process of bonding the fibers together, the binder also tends to adhere the preform to the tooling. In the past, screen tooling has been coated with a release agent or a semi-permanent coating such as polytetrafluoroethylene (PTFE, or TEFLON(copyright)) to inhibit the preform from adhering to the tooling. While suitable for many applications, release agents must typically be reapplied after each preform operation and may adversely affect the properties of the preform. TEFLON(copyright) coatings are not sufficiently durable to survive numerous molding operations, and therefore require significant production downtime to repair the coating or completely recoat the screen tooling.
In view of the above, it would be desirable if an improved screen tooling and coating system were available that was more durable, reduced or eliminated the requirement for release agents, and extended the service life of the tooling.
The present invention is directed to an improved screen tooling for a fiber preforming process, and more particularly to a durable coating system for such tooling. The coating system of this invention comprises a surface layer of a porcelain enamel composition that is generally a borosilicate glass, and preferably contains quartz (SiO2), dehydrated borax (Na2B4O7), boric acid (H3BO3), potassium nitrate (KNO3), sodium silicofluoride (Na2SiF6), and manganese dioxide (MnO2), and optionally contains titanium dioxide (TiO2), antimony oxide (Sb2O3), cobalt oxide [cobaltous oxide (CoO), cobalto-cobaltic oxide (Co3O4) and/or cobaltic oxide (Co2O3)] and/or barium oxide (BaO). Preferred compositions are dependent in part on the screen tooling material. The invention also encompasses a fiber preforming process that utilizes screen tooling with the coating system.
According to the present invention, continuous coatings formed of porcelain enamel compositions have been shown to be more durable than semi-permanent coatings currently in use, and therefore require less maintenance such that production cost and downtime are reduced. In addition, it has been determined that preforms release more readily and cleanly from screening tooling protected by the coating system of this invention, such that the quality of the fiber preform and therefore the final fiber-reinforced article is promoted. The use of release agents can be reduced or eliminated with the coating system, thereby decreasing the preform process cycle time and reducing the likelihood that the release agent will degrade the preform properties.
Other objects and advantages of this invention will be better appreciated from the following detailed description.